Add 'gdk env' subcommand

This MR adds a new sub-command gdk env. The motivation for this subcommand is to make it easier to work with GOPATH inside GDK. There are two invocations:

• $(gdk env) sets environment variables in the current shell
• gdk env SOME COMMAND runs SOME COMMAND with a modified environment. For example, gdk env vim or gdk env atom .

flatmac:gitlab-workhorse jacobvosmaer$ go test ./internal/objectstore
internal/objectstore/prometheus.go:3:8: cannot find package "github.com/prometheus/client_golang/prometheus" in any of:
	/usr/local/go/src/github.com/prometheus/client_golang/prometheus (from $GOROOT)
	/Users/jacobvosmaer/go/src/github.com/prometheus/client_golang/prometheus (from $GOPATH)
internal/objectstore/multipart.go:15:2: cannot find package "github.com/sirupsen/logrus" in any of:
	/usr/local/go/src/github.com/sirupsen/logrus (from $GOROOT)
	/Users/jacobvosmaer/go/src/github.com/sirupsen/logrus (from $GOPATH)
internal/objectstore/multipart.go:17:2: cannot find package "gitlab.com/gitlab-org/gitlab-workhorse/internal/helper" in any of:
	/usr/local/go/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/helper (from $GOROOT)
	/Users/jacobvosmaer/go/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/helper (from $GOPATH)
flatmac:gitlab-workhorse jacobvosmaer$ $(gdk env)
(in /Users/jacobvosmaer/Desktop/gitlab-development-kit)
flatmac:gitlab-workhorse jacobvosmaer$ go test ./internal/objectstore
ok  	gitlab.com/gitlab-org/gitlab-workhorse/internal/objectstore	(cached)

If we merge this we need to do a new gem release.